Health Concerns
- Ears
- EHV-1
- Biosecurity
The May and June issues of Horse Sense listed questions regarding dermatology
problems in the ear of our reader’s horses.
These lesions are referred to as aural (ear) plaques. The most
common causes are eosinophilic granulomas caused by hypersensitivity reactions
to simulian or gnat bites. These May through October insects are able to get
through all but the finest mesh head screens and just a few bites a month can
stimulate these painful lesions. The sores are tan to white and slightly raised.
They often are crumbly and can be rubbed off by hand leaving a raw surface.
Early in the season you will see small black crusts. These are dried blood and
serum from the bite sites. You can place them on a white Kleenex or paper towel
and moisten with water. They will fade red if it is blood from gnat bites. You
can apply antibiotic ointments to the raw areas and using a steroid based
ointment to eliminate some of the discomfort. It would be necessary to prevent
even one gnat bite to get complete and rapid recovery. I respect that this is
very hard to accomplish. The next causes for aural plaques are fungal and
bacterial infections. We also see them caused by viruses. Some are sarcoids in
the ear. We believe these are also a viral origin. Young horses can get warts on
the ear, although the lips and muzzle are often also involved. Your veterinarian
may be able to recognize the cause from its appearance, but a biopsy is the way
to identify the origin of the problem. There is not enough loose skin to allow
your vet to cut the lesion out and sew the edges together.
Your veterinarian may advise you to have them give your horse something for
pain before removing, treating, or taking a biopsy of the ear. Follow-up
treatments by the owner may be difficult if the horse has developed an aversion
to having the ears handled. There is a veterinarian from England at the
university that has several tips for getting your horse over the tossing of the
head and rearing. Early application of insect repellents (caution to keep out of
eyes) and use of fly masks helps to prevent many of these. I suspect insects may
be the vector (transmit) infections to healthy horses.
Try and get to these before they get real bad. Some of
my clients have mixed permetherin and heavy salves such as icthammal and applied
them to the inner surface of the pinna (ear) to keep the gnats from biting.
Unfortunately this tends to collect dirt and runs down into the ear. The gnats
breed in wet areas. Ask the DNR before treating water for gnats.
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There is a great deal of justified concern about the spread of equine herpes
virus. I want to let you know what is happing with this disease so that you can
make knowledgeable choices with your veterinarian.
Equine herpes virus type -1 (EHV-1) myeloencephalopathy
is the disease we are talking about. The Flu / Rhino vaccine you are all
familiar with is a vaccine against influenza and Equine herpes virus type – 4
(EHV-4). This is a very important vaccine for all foals and horses that are less
than four years old, that travel, show, or are stabled. It causes the familiar
fever, nasal discharge, and cough.
EHV-1 is one of the more important contagious infectious
pathogens in horses. EHV-1 can cause abortions in pregnant mares, early neonatal
death in foals, respiratory disease, and occasionally the neurologic disease
that I referred to as myeloencephalopathy.
Clinical signs of the neurologic form include ataxia (in
coordination), and paresis (inability to stand). Horses might be unable to
urinate or may frequently dribble small volumes of urine (incontinence). Horses
might have difficulty producing manure. Horses can develop cellulites
(inflammation or swelling of the limbs), and petechia (small hemorrhages of the
gums). The virus induces a biphasic fever where the first fever spike is
associated with nasal shedding of the virus, and the second fever with the
viremic phase of the infection. Horses shed the virus through the air and via
nasal secretions. The virus is also spread by contaminated hands, water, feed,
bits, and equipment. The disease has been fatal in several states.
You should isolate horses for at least 21 days after
potential exposure to EHV-1 virus. Infected horses cab shed the virus for weeks.
They should be quarantined for 30 days after the last evidence of disease. To
disinfect area you must remove all organic material first. Then use bleach at
10% strength (0ne part bleach to 9 parts water. Always add the bleach to the
water, not the water to the bleach.) You can also use phenolic-based
disinfectants. Segregate horses into the smallest possible groups. Large groups
of horses sharing a common airspace might all be infected by one horse shedding
the virus. Take temperatures daily and isolate any horse with a fever (temp.
grater that 101 degrees F) and call your vet. Don’t be embarrassed to place a
shallow tub of disinfectant at the entrance to the barn and ask visitors to step
in it before entering. Don’t let it freeze. It won’t work and people could slip.
Change the water daily, and don’t share stalls or tack among horses.
Now for those of us that just have to know why this or
that happens: It is a single gene mutation at the polymerase region of the virus
that has allowed it to replicate 10 to 100 times faster than normal. Therefore,
we get a higher viremia and shedding of the virus. The neurologic signs are
occurring during the viremic phase of the disease. It is proposed that the virus
also has an affinity for the nervous tissue (neurotropism). This has yet to be
proven. The blood-brain barrier of the horse protects the central nervous system
(CNS). This virus is causing inflammation of these endothelial cells of the
spinal column.
The standard opinion is that horses do not develop good
cross- protection between EHV-1 & EHV-4 until four years of age. Some recent
studies challenge this. The original killed virus vaccines provide a higher
antigen load and stimulate the humoral immune system. The recommended modified
live vaccine promotes a strong cell- mediated immune response with humoral
immunity. The EHV-1 virus is able to hide in the lymphocyte cells and spread
cell to cell avoiding the humoral antibody immune surveillance of the horse’s
system. Thus, the modified live vaccine is showing very good protection from
neurologic disease and decreasing the shed of the virus to other horses.
Remember we still don’t want to lose protection from EHV-4.
This spring we will be trying to stimulate “both arms” of the immune system
by timing the vaccinations to the horse. In other diseases giving the patient a
modified live vaccine and then administering a killed product 2 to 4 weeks later
can produce a prime-boost effect yielding higher protective titers and longer
immunity. As we have stated, herpes virus only establishes 90 days of protective
immunity. We will be studying the ability to improve and extend the protection
of horses to this new mutation of the Equine herpes virus.
At this point we are not recommending the use of the
modified live vaccine during an outbreak of EHV-1 on a farm. The spread of the
virus is so rapid that asymptomatic horses could be incubating the virus, and we
do not know the potential risk of immune mediated vasculitis of the CNS tissues.
We will be working closely with vaccine manufactures to help you make the best
decisions for your horse.
Again, we thank you for helping us keep our patients, your horses, healthy.
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Jeff S. Johnson DVM
